| Gastornis | |
|---|---|
 | |
| Mounted skeleton ofGastornis giganteus, sometimes referred to asDiatryma gigantea | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | †Gastornithiformes |
| Family: | †Gastornithidae Fürbringer, 1888 |
| Genus: | †Gastornis Hébert, 1855 (vide Prévost, 1855) |
| Type species | |
| †Gastornis parisiensis Hébert, 1855 | |
| Other species | |
| Synonyms | |
| |
Gastornis is an extinctgenus of large,flightless birds that lived during the mid-Paleocene to mid-Eocene epochs of thePaleogene period. Mostfossils have been found in Europe, and some species typically referred to the genus are known from North America and Asia. Several genera, including the well-studied genusDiatryma, have historically been considered junior synonyms ofGastornis. However, this interpretation has been challenged recently, and some researchers currently considerDiatryma to be a valid genus.
Gastornis species were very large birds that were traditionally thought to have been predators of various smallermammals, such as ancient, diminutiveequids. However, several lines of evidence, including the lack of hookedclaws (in knownGastornis footprints), studies of theirbeak structure andisotopic signatures of their bones, have caused scientists to now consider that these birds were probablyherbivorous, feeding on tough plant material and seeds.Gastornis is, generally, agreed to be related to theGalloanserae, the group containingwaterfowl andgamebirds.
Gastornis was first described in1855 from a fragmentaryskeleton. It was named afterGaston Planté, described as a "studious young man full of zeal", who had discovered the first fossils inclay (Argile Plastique [fr]) formation deposits atMeudon, near Paris.[1] The discovery was notable, due to the large size of the specimens, and because, at the time,Gastornis represented one of the oldest known birds.[2] Additional bones of the first known species,G. parisiensis, were found in the mid-1860s. Somewhat more-complete specimens, then referred to the new speciesG. edwardsii (now considered asynonym ofG. parisiensis), were found a decade later. These specimens, found in the1870s, formed the basis for a widely- circulated and reproduced skeletal restoration byLemoine. The skulls of these originalGastornis fossils were unknown, other than nondescript fragments and several bones used in Lemoine's illustration, which turned out to be those of other animals.[3] Thus, this European specimen was long reconstructed as a sort of gigantic "crane-like" bird.[4][5]
In1874, the AmericanpaleontologistEdward Drinker Cope discovered another fragmentary set of fossils at theWasatch Formation,New Mexico. Cope considered the fossils to be of a distinct genus and species of giant ground bird; in1876, he named the remainsDiatryma gigantea (/ˌdaɪ.əˈtraɪmə/DY-ə-TRY-mə),[6] from theAncient Greek διάτρημα (diatrema), meaning "through a hole", in reference to the large foramina (perforations) that penetrated some of the foot bones.[7][8] In1894, a singlegastornithidtoe bone fromNew Jersey was described by Cope's "rival"Othniel Charles Marsh, and classified as a new genus and species:Barornis regens. In1911, it was recognized that this, too, could be considered a junior synonym ofDiatryma (and therefore, later,Gastornis).[9] Additional, fragmentary specimens were found inWyoming in 1911, and assigned (in1913) to the new speciesDiatryma ajax (also now considered a synonym ofG. giganteus).[9] In1916, anAmerican Museum of Natural History expedition to theBighorn Basin (Willwood Formation, Wyoming) found the first nearly-complete skull and skeleton, which was described in1917 and gave scientists their first clear picture of the bird.[9] Matthew, Granger, and Stein (1917) classified this specimen as yet another new species,Diatryma steini.[9]
After the description ofDiatryma, most new European specimens were referred to this genus, instead ofGastornis; however, after the initial discovery ofDiatryma, researchers recognized the similarity between the two genera as early as1884 whenElliott Coues placedDiatryma gigantea under the genusGastornis asG. giganteus, a synonymy agreed upon by the American Ornithologists' Union in1886.[10] Further meaningful comparisons betweenGastornis andDiatryma were made more difficult by Lemoine's incorrect skeletal illustration, the composite nature of which was not discovered until the early1980s. Following this, several authors began to recognize a greater degree of similarity between the European and North American birds, often placing both in the same order (†Gastornithiformes) or even family (†Gastornithidae). This newly-realized degree of similarity caused many scientists to, tentatively, accept the animals' synonymy pending a comprehensive review of the anatomy of both genera, in whichGastornis has the taxonomic priority.[2] Some subsequent studies either continued to use the genusDiatryma or argued against the synonymy, since a detailed comparison of type specimens has not been done yet and notable differences can be found in the species originally assigned toDiatryma from the type species ofGastornis.[11][12]
Gastornis is known from a large amount of fossil remains, but the clearest picture of the bird comes from a few nearly complete specimens of the speciesG. giganteus. These were generally very large birds, with huge beaks and massive skulls superficially similar to the carnivorous South American "terror birds" (phorusrhacids). The largest known species,G. giganteus would have reached about 2 m (6 ft 7 in) in maximum height,[13] and up to 175 kg (386 lb) in mass.[14]
The skull ofG. giganteus was huge compared to the body and powerfully built. The beak was extremely tall and compressed (flattened from side to side). Unlike other species ofGastornis,G. giganteus lacked characteristic grooves and pits on the underlying bone. The 'lip' of the beak was straight, without a raptorial hook as found in the predatory phorusrhacids. The nostrils were small and positioned close to the front of the eyes about midway up the skull. The vertebrae were short and massive, even in the neck. The neck was relatively short, consisting of at least 13 massive vertebrae. The torso was relatively short. The wings were vestigial, with the upper wing-bones small and highly reduced, similar in proportion to the wings of thecassowary.[9] A largely complete skull specimen (GMH XVIII-1178-1958) ofG. geiselensis was also described in 2024 after its discovery in 1958. The upper beaks ofG. geiselensis show possible sexual dimorphism and are wider than those ofG. giganteus and proportionally longer than those ofG. laurenti.[12]
Gastornis and its close relatives are classified together in thefamilyGastornithidae, and were long considered to be members of the orderGruiformes. However, the traditional concept of Gruiformes has since been shown to be anunnatural grouping. Beginning in the late 1980s with the firstphylogenetic analysis of gastornithid relationships, consensus began to grow that they were close relatives of the lineage that includeswaterfowl andscreamers, theAnseriformes.[11] A 2007 study showed that gastornithids were a very early-branching group of anseriformes, and formed the sister group to all other members of that lineage.[15]
Recognizing the apparent close relationship between gastornithids and waterfowl, some researchers classify gastornithids within the anseriform group itself.[15] Others restrict the name Anseriformes only to thecrown group formed by all modern species, and label the larger group including extinct relatives of anseriformes, like the gastornithids, with the nameAnserimorphae.[16] Gastornithids are therefore sometimes placed in their ownorder,Gastornithiformes.[17] A 2024 study, however, found little support for Gastornithiformes and instead placesGastornis as a member of the Galliformescrown group, as more closely related toPhasianoidea than tomegapodes, being sister to the extinctSylviornithidae, a recently extinct group of medium-sized flightless birds known from subfossil deposits in the Western Pacific.[18]
A simplified version of the family tree found by Agnolinet al. in 2007 is reproduced below.
| Anseriformes |
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As of 2024, at least three species are confidently placed within the genusGastornis:G. parisiensis (type species),G. russelli andG. laurenti.[12] Thetype species,Gastornis parisiensis, was named and described by Hébert in two 1855 papers.[19][20] It is known from fossils found in western and central Europe, dating from the late Paleocene to the early Eocene. Other species previously considered distinct, but which are now considered synonymous withG. parisiensis, includeG. edwardsii (Lemoine, 1878) andG. klaasseni (Newton, 1885). Additional European species ofGastornis areG. russelli (Martin, 1992) from the late Paleocene ofBerru, France, andG. sarasini (Schaub, 1929) from the early-middle Eocene.[3] The supposed small speciesG. minor is considered to be anomen dubium.[2]
Named in 2020,G. laurenti is the most recently described species ofGastornis from southwestern France. The holotype (MHNT.PAL.2018.20.1) is a nearly complete mandible which differs from other species within the genus, and the paratypes consist of the maxilla, right quadrate, femur shaft, tibiotarsus (two left and one right) and six cervical vertebrae.[21] A 2024 study attributed more postcranial remains from the same locality toG. laurenti.[22]
Gastornis giganteus (Cope, 1876), formerlyDiatryma gigantea, dates from the middle Eocene of western North America. Its junior synonyms includeBarornis regens (Marsh, 1894) and possiblyOmorhamphus storchii (Sinclair, 1928).O. storchii was described based on fossils from lowerEocene rocks ofWyoming.[23] The species was named in honor of T. C. von Storch, who found the fossils remains in Princeton 1927 Expedition.[24] The fossil bones originally described asOmorhamphus storchii are considered to be the remains of a juvenileGastornis giganteus by Brodkorb (1967),[25] but Louchart et al. (2021) argued that no definitive juvenile specimens ofG. giganteus are known and that the two taxa have no known association, so there is no unambiguous evidence to support this synonymy.[26] SpecimenYPMPU 13258 from lower EoceneWillwood Formation rocks ofPark County, Wyoming also seems to be a juvenile – perhaps also ofG. giganteus, in which case it would be an even younger individual.[27]
G. geiselensis, from the middle Eocene ofMessel, Germany, has been considered a synonym ofG. sarasini;[28] however, other researchers have stated that there is currently insufficient evidence to synonymize the two, and that they should be kept separate at least pending a more detailed comparison of all gastornithids.[29]
In 2024,Gerald Mayr and colleagues argued against the synonymy ofDiatryma withGastornis based on the distinct features of the coracoid and tarsometatarsus ofG. giganteus andG. geiselensis, referred to asD. gigantea andD. geiselensis in the paper, when compared to those ofG. parisiensis. They further suggested that these two features support the placement ofG. sarasini withinDiatryma asD. sarasini, and that assigning all species of gastornithiforms to the genusGastornis would not properly reflect the interrelationships of this taxonomic group. A simplified version of their phylogenetic analysis is reproduced below:[12]
| Gastornithiformes |
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A tibiotarsus (upper foot bone) originally described in 1980 asZhongyuanus xichuanensis from the early Eocene ofHenan, China,[30] was suggested to be an Asian species ofGastornis in 2013.[31] However, the 2024 study which argued against the synonymy ofDiatryma withGastornis suggested that this fragmentary Chinese taxon cannot be confidently assigned to eitherDiatryma orGastornis, and thus more evaluation is required to clarify its taxonomic affinities.[12]
A long-standing debate surroundingGastornis is the interpretation of its diet. It has often been depicted as a predator of contemporary small mammals, which famously included the early horseEohippus.[9] However, with the size ofGastornis legs, the bird would have had to have been more agile to catch fast-moving prey than the fossils suggest it to have been. Consequently,Gastornis has been suspected to have been an ambush hunter and/or used pack hunting techniques to pursue or ambush prey; ifGastornis was a predator, it would have certainly needed some other means of hunting prey through the dense forest. Alternatively, it could have used its strong beak for eating large or strong vegetation.
The skull ofGastornis is massive in comparison to those of livingratites of similar body size.Biomechanical analysis of the skull suggests that the jaw-closing musculature was enormous. The lower jaw is very deep, resulting in a lengthened moment arm of the jaw muscles. Both features strongly suggest thatGastornis could generate a powerful bite.[13] Some scientists have proposed that the skull ofGastornis was 'overbuilt' for a herbivorous diet and support the traditional interpretation ofGastornis as a carnivore that used its powerfully constructed beak to subdue struggling prey and crack open bones to extract marrow.[13] Others have noted the apparent lack of predatory features in the skull, such as a prominently hooked beak, as evidence thatGastornis was a specialized herbivore (or even anomnivore) of some sort, perhaps having used its large beak to crack hard foods like nuts and seeds.[32] Footprints attributed to gastornithids (possibly a species ofGastornis itself), described in 2012, showed that these birds lacked strongly hooked talons on the hind legs, another line of evidence suggesting that they did not have a predatory lifestyle.[33]
Recent evidence suggests thatGastornis was likely a true herbivore.[14] Studies of the calcium isotopes in the bones of specimens ofGastornis by Thomas Tutken and colleagues showed no evidence that it had meat in its diet. The geochemical analysis further revealed that its dietary habits were similar to those of both herbivorous dinosaurs and mammals when it was compared to known fossil carnivores, such asTyrannosaurus rex, leavingphorusrhacids andbathornithids as the only major carnivorous flightless birds.[34] The first in situ preservedgastroliths in a specimen ofG. geiselensis (orD. geiselensis) also conforms to its herbivorous diet.[12]
InLate Paleocene deposits of Spain and earlyEocene deposits of France, shell fragments of hugeeggs have turned up, namely inProvence.[35][36] These were described as theootaxonOrnitholithus and are presumably fromGastornis. While no direct association exists betweenOrnitholithus andGastornis fossils, no other birds of sufficient size are known from that time and place; while the largeDiogenornis andEremopezus are known from the Eocene, the former lived in South America (still separated from North America by theTethys Ocean then) and the latter is only known from the Late Eocene of North Africa, which also was separated by an (albeit less wide) stretch of the Tethys Ocean from Europe.[37]
Some of these fragments were complete enough to reconstruct a size of 24 by 10 cm (about 9.5 by 4 inches) with shells 2.3–2.5 mm (0.09–0.1 in) thick,[35] roughly half again as large as an ostrich egg and very different in shape from the more rounded ratite eggs. IfRemiornis is indeed correctly identified as a ratite (which is quite doubtful, however[28]),Gastornis remains as the only known animal that could have laid these eggs. At least one species ofRemiornis is known to have been smaller thanGastornis, and was initially described asGastornis minor by Mlíkovský in 2002. This would nicely match the remains of eggs a bit smaller than those of the living ostrich, which have also been found inPaleogene deposits of Provence, were it not for the fact that these eggshell fossils also date from the Eocene, but noRemiornis bones are yet known from that time.[36]
Several sets of fossil footprints are suspected to belong toGastornis. One set of footprints was reported from lateEocenegypsum atMontmorency and other locations of theParis Basin in the 19th century, from 1859 onwards. Described initially byJules Desnoyers, and later on byAlphonse Milne-Edwards, these trace fossils were celebrated among French geologists of the late 19th century. They were discussed byCharles Lyell in hisElements of Geology as an example of the incompleteness of the fossil record – no bones had been found associated with the footprints.[38] Unfortunately, these fine specimens, which sometimes even preserved details of the skin structure, are now lost. They were brought to theMuséum national d'histoire naturelle when Desnoyers started to work there, and the last documented record of them deals with their presence in the geology exhibition of the MNHN in 1912. The largest of these footprints, although only consisting of a single toe's impression, was 40 cm (16 in) long. The large footprints from theParis Basin could also be divided into huge and merely large examples, much like the eggshells from southern France, which are 20 million years older.[37]
Another footprint record consists of a single imprint that still exists, though it has proven to be even more controversial. It was found in late EocenePuget Group rocks in theGreen River valley nearBlack Diamond, Washington. After its discovery, it raised considerable interest in theSeattle area in May–July 1992, being subject of at least two longer articles in theSeattle Times.[39][40] Variously declared ahoax or genuine, this apparent impression of a single bird foot measures about 27 cm (11 in) wide by 32 cm (13 in) long and lacks ahallux (hind toe); it was described as theichnotaxonOrnithoformipes controversus. Fourteen years after the initial discovery, the debate about the find's authenticity was still unresolved.[41] The specimen is now atWestern Washington University.[42][43]
_type_specimen.jpg)
The problem with these early trace fossils is that no fossil ofGastornis has been found to be younger than about 45 million years. In North America, the fossil record of unequivocal gastornithids seems to end even earlier than in Europe.[37][43] However, in 2009, a landslide nearBellingham, Washington exposed at least 18 tracks on 15 blocks in the EoceneChuckanut Formation. The anatomy and age (about 53.7 Ma old[44]) of the tracks suggest that the track maker wasGastornis. Although these birds have long been considered to be predators or scavengers, the absence of raptor-like claws supports earlier suggestions that they were herbivores. The Chuckanut tracks are named as the ichnotaxonRivavipes giganteus, inferred to belong to the extinct family Gastornithidae. At least 10 of the tracks are on display at Western Washington University.[45]
Theplumage ofGastornis has generally been depicted in art as a hair-like covering similar to someratites. This has been based in part on some fibrous strands recovered from aGreen River Formation deposit atRoan Creek, Colorado, which were initially believed to representGastornis feathers and namedDiatryma? filifera.[46] Subsequent examination has shown the fossil material to not actually be feathers,[47] butroot fibers and the species renamed asCyperacites filiferus.[48]
A second possibleGastornis feather has since been identified, also from the Green River Formation. Unlike the filamentous plant material, this single isolated feather resembles the body feathers of flighted birds, being broad and vaned. It was tentatively identified as a possibleGastornis feather based on its size; the feather measured 240 mm (9.4 in) long and must have belonged to a gigantic bird.[49][50]
It has been argued thatGastornis has aHolarctic distribution with fossils found in western Europe, North America (including an indeterminate specimen identified asGastornis sp. fromArctic Canada), and possibly central China.[51] The earliest (Paleocene) fossils all come from Europe, and it is likely that the genus originated there. Europe in this epoch was an island continent, andGastornis was the largest terrestrial tetrapod of the landmass. This offers parallels with the Malagasyelephant birds, herbivorous birds that were similarly the largest land animals in the isolated landmass ofMadagascar, in spite of otherwise mammalian megafauna.[52]
All other fossil remains are from the Eocene, though it is currently unknown how the genusGastornis dispersed out of Europe and into other continents, and whether such assertion is even true given the potential validity ofDiatryma.[12] Given the possible presence ofGastornis fossils in the early Eocene of western China, these birds may have spread east from Europe and crossed into North America via theBering land bridge.Gastornis also may have spread both east and west, arriving separately in eastern Asia and in North America across theTurgai Strait.[31] Direct landbridges with North America are also known.[52] EuropeanGastornis survived somewhat longer than their North American counterparts, which seems to coincide with a period of increased isolation of the continent.[52]
The reason for the extinction ofGastornis is currently unclear. Competition with mammals has often been cited as a possible factor, butGastornis did occur in faunas dominated by mammals, and did co-exist with several megafaunal forms likepantodonts.[52] Likewise, extreme climatic events like thePaleocene–Eocene Thermal Maximum (PETM) appear to have had little impact.[52] Nonetheless, the extended survival in Europe is thought to coincide with increased isolation of the landmass.[52]
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